The present invention relates to a dual speed rotary mechanical vacuum pump. More particularly, the present invention relates to a rotary mechanical vacuum pump, for example, of the rotary vane type, which is connected to an electrical motor so that it can be selectively driven at either a high speed for roughing purposes or at a lower speed for backing a high vacuum type pump, for example, a diffusion or turbo pump.
In a typical high-vacuum system, a mechanical or roughing pump is initially connected to the chamber to be evacuated to pump same down from the atmospheric pressure to a pressure at which a high-vacuum pump, for example, a diffusion or turbo pump can operate. Thereafter further pumping down of the chamber is carried out by the high-vacuum pump. Optimally, the size of the roughing pump will be such as to provide a minimum desired pump down or roughing time. However, in such high-vacuum systems, as is well known to those skilled in the art, a further vacuum pump is required in order to provide back pressure for the high-vacuum pump, i.e., the diffusion or turbo pump, of the system. In order to provide this back pressure, either a further and separate mechanical pump may be provided or alternatively, for reasons of economy and space, the same single speed mechanical pump is used as both the roughing pump and as the backing pump. Historically when a single pump serves both of these functions, the size of the mechanical pump is a compromise between the optimum roughing speed and the required backing capacity and consequently the mechanical pump operates at a speed which is less than that required to provide the minimum roughing time but greater than that required to provide the desired backing capacity. Additionally, since in a conventional or typical vacuum process, such a mechanical vacuum pump would operate as a backing pump for a substantially greater period of its operating time, the use of such a single speed pump for both a roughing and a backing pump results in increased wear on the mechanical parts of the pump and increased power consumption, which obviously is a further disadvantage.